This invention relates to flexible polishing pads for lenses or the like, and more particularly to an improved pad which utilizes a water soluble binder or matrix for effecting controlled release of polishing particles from the pad during a polishing operation.
As noted in column 1 of U.S. Pat. No. 4,255,164, the principal steps in producing polished optical surfaces, such as for example polished surfaces on glass lenses, comprises three successive operations--namely, a rough generating step using a tool containing a coarse, hard abrasive such as diamond particles, or the like; a grinding or fining step using finer abrasive particles to remove deep scratches and to compensate for slight generating errors and to produce the desired curve on the lens itself; and a final polishing step using a compound of extremely fine particle size for removing small scratches and to provide a smooth lens surface of optical quality. This invention, it will be understood, relates to the above-noted final polishing step, and a novel pad particularly suited for use in such polishing step.
The conventional method of polishing lenses has been to employ a liquid slurry comprising, for example, very fine polishing particles in an aqueous solution. The slurry is applied to the interface between the surface of the lens and the associated polishing pad or lap. Because of the obvious inconvenience of having to employ a slurry which contains polishing particles, efforts have been made over the years to provide a satisfactory polishing pad which can be secured over a polishing lap, and which contains the necessary polishing particles. In this way only water need be applied to the pad during polishing.
U.S. Pat. No. 4,255,164, for example, discloses a flexible, glass fining sheet or pad in which abrasive particles or granules are secured in a water insoluble resinous binder, such as for example a thermosetting polymer modified by a small amount of thermoplastic polymeric latex. During the grinding (fining) operation it is then only necessary to apply water to the interface between the fining pad and the lens surface in order to create the necessary fining slurry. However, as pointed out in column 1 of this patent, a fining sheet or pad of this type is concerned only with the grinding (fining) of the lens surface. The abrasive granules employed for such purpose, therefore, are said to have a Knoop hardness of at least about 1,000 and an average particle size of about 10 to 80 microns, ranges which are not satisfactory for polishing purposes. Moreover, these abrasive particles are released from the water-insoluble binder during grinding as the result of the gradual mechanical erosion of the binder due to the effects of loading and surface friction.
For both fining and polishing operations, experience has indicated that best results are achieved when the abrasive or polishing particles are free to roll or move in the slurry generated between the lens surface and the fining or polishing pad surface. In the case of the above-noted U.S. Pat. No. 4,255,164, this release of the abrasive particles depends solely upon the mechanically induced failure of the binder matrix, rather than upon the binder solubility. In accordance with the teachings of the present invention, however, it has been found that it is not only possible, but is more desirable to produce a polishing pad containing a water-soluble binder which, during use, dissolves at a rate that permits the controlled release of the polishing particles at a predetermined rate, thus considerably increasing the quality, convenience and efficiency of the polishing operation.
Although attempts heretofore have been made to produce a polishing or grinding matrix comprising a water-soluble binder composition, the efforts have proved to be unsatisfactory because of the uncontrolled, rapid disintegration of the matrix. U.S. Pat. No. 3,042,509, for example, proposed using a water soluble matrix for abrasive particles comprising a mixture of polyethlene glycol (20-80%). Such a matrix is solid at room temperature, and has good lubricating properties during use. The problem with this type of matrix is, however, that it dissolves far too rapidly during use, and if used for polishing purposes is incapable of approaching a stock removal rate common to conventional slurry polishing techniques. By way of example, laboratory tests conducted on a conventional Coburn 505 polisher indicate that it is commonplace to achieve stock removal rates of approximately 120 mg. of glass per twelve minutes of polishing a glass lens of 55.5 mm. diameter using a conventional "Pellon" pad under 30 psig., and a slurry comprising a 5% concentration of a cerium oxide polishing compound of the type distributed by Transelco Division of Ferro Corporation under the name "Ce-Rite" Rx 419.
More recently, U.S. Pat. No. 4,138,228 has suggested incorporating a polishing abrasive having an average particle size of less than 10 microns in a microporous polymeric structure, which exists in the form of tiny platelets, rather than in the form of a monolithic film. The alleged advantage of this invention is that the abrasive particles are adhered on the surfaces of the platelets, or at the most are only slightly embedded in the platelets, so that when the abrasive surface of the pad is rubbed against the surface of a glass lens in the presence of water or the like, the combined action of the rubbing and the absorption of the liquid into the microporous or sponge-like polymer matrix effects controlled release of the polishing abrasive from the surface of the platelets. The essence of this type of pad is the fact that the particles are substantially entirely unencapsulated by the binder, so that during the polishing process they are released as the result of the mechanical activity generated during polishing.
It is an object of this invention, therefore, to provide an improved polishing pad which utilizes a water soluble matrix that is specifically designed to provide controlled, gradual release of polishing particles during a polishing operation.
Other objects of the invention wll be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.